JP2003033024A - Switching power supply unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form feedback control circuit without any involvement of an insulation element. SOLUTION: When a switching device 2 which permits a commutation diode 6 to have continuity is off, induced electromotive voltage occurring in a main winding 22 becomes equal to output voltage Vo. In a voltage detection winding 23, voltage proportional to a turn ratio to that of the main winding 22 is generated in a condition electrically insulated to the main winding 22. By using the voltage generated at the voltage detection winding 23, detected voltage dependent on fluctuations in the output voltage Vo can be supplied from an output voltage detection circuit 27 to a pulse duration control circuit 12, thereby attaining stabilizing the output voltage Vo without any involvement of an insulation element.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention includes a feedback control circuit including a low output voltage protection circuit and a high output voltage protection circuit for stabilizing an output voltage. The present invention relates to a switching power supply device. FIG. 2 is a general circuit diagram of a conventional forward-type switching power supply.
In FIG. 1, reference numeral 1 denotes a transformer for insulating the primary winding 1A from the secondary winding 1B, and 2 denotes a switching element composed of, for example, a MOS-type FET.
A series circuit of the DC power supply 3 is connected between both ends of the DC power supply 3. Reference numeral 4 denotes a rectifying / smoothing circuit connected to the secondary winding 1B of the transformer 1. The rectifying / smoothing circuit 4 includes a rectifying diode 5 provided as a rectifying element, as is well known.
And a commutation diode 6 provided as a commutation element, a smoothing choke coil 7 and a capacitor 8, and a pair of outputs for supplying an output voltage Vo to a load 9 between both ends of the capacitor 8. Terminals 10 and 11 are connected. As a feedback circuit for stabilizing the output voltage Vo, there is provided a pulse width control circuit 12 corresponding to a feedback control circuit for variably controlling the pulse conduction width of the switching element 2 according to the fluctuation of the output voltage Vo. .
The pulse width control circuit 12 compares a detection voltage obtained by dividing the output voltage Vo with a built-in reference voltage, and outputs a result of each ratio. Based on the comparison result obtained by the circuit 13, the switching element 2
A primary-side control circuit 14 that varies the pulse conduction width of the drive signal supplied to the primary-side control circuit 14, and an insulating element 15 such as a photocoupler that electrically insulates signal transmission from the secondary-side control circuit 13 to the primary-side control circuit 14. It consists of. In addition, although not shown here, the feedback control circuit includes a low output voltage protection circuit and a high output voltage protection circuit for protecting the power supply device. In the above configuration, when the DC voltage Vi from the DC power supply 3 is intermittently applied to the primary winding 1A of the transformer 1 by the switching operation of the switching element 2,
The AC voltage extracted from the secondary winding 2 of the transformer 1 is rectified and smoothed by the rectifying and smoothing circuit 4, and a DC output voltage Vo is supplied from between both ends of the smoothing capacitor 8 to both ends of the load 9.
Specifically, the switching element 2 is turned on by a pulse-on signal from the primary side control circuit 14 constituting the pulse width control circuit 12.
Is turned on, a positive voltage is generated at the dot-side terminal of the secondary winding 1B of the transformer 1, and the rectifier diode 5 is turned on, while the commutation diode 6 is turned off. As a result, energy is supplied from the secondary winding 1B of the transformer 1 to the choke coil 7 and the load 9 through the rectifier diode 5. Thereafter, when the switching element 2 is turned off by the pulse-off signal from the primary side control circuit 14, the commutation diode 6 is turned on while the rectifier diode 5 is turned off. As a result, the energy that has been stored in the choke coil 7 until then is supplied to the load 9 through the commutation diode 6. The smoothing capacitor 8 absorbs the ripple of the output voltage Vo. A secondary side control circuit 13 constituting the pulse width control circuit 12 compares a detection voltage obtained by dividing the output voltage Vo with a reference voltage, and outputs a voltage corresponding to the comparison result to an insulating element. The signal is transmitted to the primary-side control circuit 14 via 15. When the output voltage Vo is increasing, the primary side control circuit 14 narrows the conduction width of the drive signal to the switching element 2 based on the comparison result from the secondary side control circuit 13, and conversely, the output voltage V
When o decreases, the conduction width of the drive signal to the switching element 2 is increased to stabilize the output voltage Vo. The pulse width control circuit 12 configured as described above detects an output voltage on the secondary side of the transformer 1 by a voltage dividing resistor, and outputs a pulse drive signal to the switching element 2 on the primary side of the transformer 1 based on the detected voltage. Due to the control, the primary side control circuit 14 and the secondary side control circuit 13 must be divided into two parts, and the insulating element 15 must be interposed between them. That is, without the insulating element 15, an accident such as a short circuit occurring on the secondary side of the transformer 1 may spread to the primary side of the transformer 1 through the pulse width control circuit 12, which is a feedback circuit. However, the use of such an insulating element not only lowers the reliability of the power supply device, but also increases the number of components and the cost. SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a switching power supply device capable of forming a feedback control circuit without an insulating element. In the switching power supply according to the present invention, when the switching element is turned on, the rectifying element conducts and supplies energy from the secondary winding of the transformer to the smoothing choke coil. When the switching element is turned off, the commutation element conducts and sends out the energy of the choke coil to the load side, and the pulse supplied to the switching element so that the output voltage supplied across the load is stabilized. In a switching power supply device having a feedback control circuit for controlling a drive signal, a voltage detection winding is wound around the choke coil while being insulated from a main winding, and is generated in the voltage detection winding when the commutation element is turned on. An output voltage detection circuit that supplies a detection voltage to the feedback control circuit using a voltage. It is. In this case, when the switching element is turned on, the rectifying element is turned on to supply energy from the secondary winding of the transformer to the main winding of the choke coil, while when the switching element is turned off, the commutation element is turned on. The energy stored in the choke coil until then is sent to the load. In particular, when the switching element in which the commutation element conducts is off, the induced electromotive voltage generated in the main winding of the choke coil becomes equal to the output voltage. A voltage proportional to the turns ratio is generated while being electrically insulated from the main winding.
Therefore, if the voltage generated in the voltage detection winding at this time is used, the detection voltage depending on the fluctuation of the output voltage can be supplied from the output voltage detection circuit to the feedback control circuit, and an insulating element such as a photocoupler can be provided. The feedback control circuit can be configured without bothering the user. Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. The same parts as those shown in FIG.
The detailed description of the common part is omitted because it is duplicated. FIG. 1 is a circuit diagram of a forward-type switching power supply according to an embodiment of the present invention. In FIG. 1, a choke coil 21 in this embodiment is inserted and connected to a positive or negative DC voltage line. In addition to the main winding 22, a voltage detection winding 23 wound separately from the main winding 22 is provided. This choke coil 21
And the voltage supplied through this diode 25, including the voltage detection winding 23, which conducts when the commutation diode 6 conducts and an induced electromotive force is generated in the main winding 22 of the choke coil 21. The charging voltage generated between both ends of the capacitor 26 by the charging capacitor 26 is output voltage Vo
An output voltage detection circuit 27 to be supplied to the pulse width control circuit 12, a low output voltage protection circuit (not shown), and a high output protection circuit (not shown) as the detection voltage of. Specifically, when an induced electromotive force is generated in the main winding 22, the diode 25
A series circuit of the capacitor 26 and the voltage detection winding 23 is connected between both ends of the voltage detection winding 23. And a connection point between the diode 25 where the charging voltage of the capacitor 26 is generated and the capacitor 26 is connected to the input side of the pulse width control circuit 12. The pulse width control circuit 12 in the present embodiment includes a main winding 22.
Because the voltage detection winding 23 and the voltage detection winding 23 are electrically insulated from each other, the primary side control circuit 14 and the secondary side control circuit 13 are not partitioned as in the conventional example. Therefore, no insulating element for signal transmission is used inside the pulse width control circuit 12. Next, the operation of the above configuration will be described. When the switching element 2 is turned on by a pulse-on signal from the pulse width control circuit 12, a DC input voltage Vi is applied to the primary winding 1A of the transformer 1, and Next winding 1
A positive voltage is generated at the dot side terminal of B, and the commutation diode 6 is turned off while the rectifier diode 5 is turned on. As a result, energy is supplied from the secondary winding 1B of the transformer 1 to the choke coil 21 and the load 9 through the rectifier diode 5. Thereafter, when the switching element 2 is turned off by the pulse-off signal from the primary side control circuit 14, a positive voltage is generated at the non-dot side terminal of the primary winding 1B of the transformer 1, and the rectifier diode 5 is turned off, The current diode 6 turns on. As a result, the energy previously stored in the choke coil 21 is supplied to the load 9 through the commutation diode 6. that time,
The smoothing capacitor 8 absorbs the ripple of the output voltage Vo. The above operation is the same as the conventional example. The switching element 2 is turned on, the rectifier diode 5 is turned on, and the main winding 21 of the choke coil 21 and the load 9 are turned on.
While energy is supplied to the main winding 22, a positive voltage is generated at the dot side terminal of the main winding 22 and thus the voltage detection winding 23, so that the diode 25 constituting the output voltage detection circuit 27 is in a non-conductive state. That is, since the voltage generated between both ends of the voltage detection winding 23 is not proportional to the output voltage Vo, the output voltage detection circuit 27 is interposed between the voltage detection winding 23 and the capacitor 26. The diode 25 is turned off, and charging of the capacitor 26 is cut off. On the other hand, when the switching element 2 is turned off and the commutation diode 6 is turned on to generate an induced electromotive force in the main winding 22 of the choke coil 21, the non-dot side terminal of the voltage detection winding 23 A voltage is generated, and the diode 26 constituting the output voltage detection circuit 27 conducts. At this time, the output voltage Vo is applied between both ends of the main winding 21 as it is, so that the output voltage detection circuit 27
Charge. As a result, the following charging voltage Vc is generated in the capacitor 26. ## EQU1 ## In the above equation (1), N1 is the number of turns of the main winding 22, N2 is the number of turns of the voltage detecting winding 23, Vo is the output voltage, V
f is the forward voltage drop of the diode 25. The charging voltage Vc of the capacitor 26 thus obtained depends on the output voltage Vo, and the charging voltage Vc is supplied from the output voltage detecting circuit 27 to the pulse width control circuit 12 as a detection voltage of the output voltage Vo. . When the output voltage Vo and, consequently, the charging voltage Vc of the capacitor 26 rises, the pulse width control circuit 12 narrows the conduction width of the drive signal to the switching element 2, and conversely, when the output voltage Vo and thus the charging voltage Vc of the capacitor 26 decreases, Switching element 2
To increase the conduction width of the drive signal. At this time, since the main winding 22 and the voltage detection winding 23 are electrically insulated, the output voltage Vo is stabilized without interposing an insulating element such as a photocoupler in the pulse width control circuit 12. be able to. As described above, according to the present embodiment, when the switching element 2 is turned on, the rectifier diode 5, which is a rectifier, conducts, and energy is transferred from the secondary winding 1B of the transformer 1 to the smoothing choke coil 21. When the switching element 2 is turned off, the commutation diode 6 which is a commutation element is supplied.
Is conducted, the energy of the choke coil 21 is sent out to the load 9 side, and the output voltage V supplied between both ends of the load 9 is output.
In a switching power supply device provided with a pulse width control circuit 12 as a feedback control circuit for controlling a pulse drive signal supplied to the switching element 2 so that o is stabilized, the choke coil 21 is insulated from the main winding 22. And a feedback control circuit (a low output voltage protection circuit, a high output voltage protection circuit, a pulse width control circuit) utilizing the voltage generated in the voltage detection winding 23 when the commutation diode 6 is turned on. The detection voltage (charging voltage Vc) is applied to the circuit 12).
And an output voltage detection circuit 27 that supplies the output voltage. In this case, when the switching element 2 is turned on, the rectifier diode 5 is turned on to supply energy from the secondary winding 1B of the transformer 1 to the main winding 22 of the choke coil 21, while the switching element 2 is turned off. Then, the commutation diode 6 becomes conductive, and the energy stored in the choke coil 21 until then is sent to the load 9. And
In particular, when the switching element 2 in which the commutation diode 6 conducts is off, the induced electromotive voltage generated in the main winding 22 of the choke coil 21 becomes equal to the output voltage Vo. A voltage proportional to the turn ratio with the main winding 22 is generated in a state where the voltage is electrically insulated from the main winding 22. Therefore, if the voltage generated in the voltage detection winding 23 at this time is used, the detection voltage depending on the fluctuation of the output voltage Vo can be supplied from the output voltage detection circuit 27 to the pulse width control circuit 12 and the like. The low output voltage protection circuit, the high output voltage protection circuit, and the pulse width control circuit 12 can be configured without intervening an insulating element such as a coupler. For this reason, it is possible to eliminate a decrease in reliability due to the use of the insulating element, and an increase in the number of parts, and thus an increase in cost, which have been concerned. As an effect of the present embodiment, the output voltage detecting circuit 27 of the present embodiment is configured such that, in addition to the voltage detecting winding 23, the commutation diode 6 conducts and is induced in the main winding 22 of the choke coil 21. It comprises a diode 25 that conducts when an electromotive force is generated, and a capacitor 26 that charges a voltage supplied through the diode 25. The charging voltage Vc generated between both ends of the capacitor 26 is used as a detection voltage of the output voltage Vo and a pulse width It is configured to supply to the control circuit 12. In this manner, while the rectifier diode 5 is conducting, the diode 25 is in a non-conducting state and the charging of the capacitor 26 is cut off. Conduction is performed, and the capacitor 26 is charged. Thus, the capacitor 26 is charged only during the period when the voltage between both ends of the main winding 22 is equal to the output voltage Vo, and the output voltage detection circuit 27 can accurately detect the output voltage Vo. Moreover, the output voltage detection circuit 27 is composed of the voltage detection winding 23, the diode 25, and the capacitor 26.
Therefore, the above-described operation and effect can be realized with a simple circuit configuration. It should be noted that the present invention is not limited to the above embodiment, and various modifications can be made. For example, although a diode is used as the rectifying element or the commutating element in the embodiment, a field effect transistor (MOS type FET) that turns on and off in synchronization with the switching element 2 may be used. In this case, the voltage effect when the commutation MOS FET is turned on is reduced, and the output voltage detection circuit 27 can more accurately detect the output voltage Vo without being affected by the characteristics of the commutation MOS FET. become. Further, in this embodiment, the pulse width control circuit realizing the PWM control is used as the feedback control circuit, but the PFM control for controlling the frequency of the pulse drive signal supplied to the switching element or the PWM control and the PFM control are combined. A control circuit may be used. According to the switching power supply of the present invention, when the switching element is turned on, the rectifying element conducts and supplies energy from the secondary winding of the transformer to the smoothing choke coil. When turned off, the commutation element conducts and sends out the energy of the choke coil to the load side, and controls the pulse drive signal supplied to the switching element so that the output voltage supplied between both ends of the load is stabilized. In a switching power supply device having a feedback control circuit, a voltage detection winding is wound around the choke coil while being insulated from a main winding, and a voltage generated in the voltage detection winding when the commutation element is turned on is used. And an output voltage detection circuit for supplying a detection voltage to the feedback control circuit. The output voltage can be stabilized by the feedback control circuit without intervening such an insulating element.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a switching power supply device according to an embodiment of the present invention. FIG. 2 is a circuit diagram of a switching power supply device in a conventional example. [Description of Signs] 1 Transformer 2 Switching element 5 Rectifying diode (Rectifying element) 6 Commutating diode (Commutating element) 9 Load 12 Pulse width control circuit (Feedback control circuit) 21 Choke coil 22 Main winding 23 Voltage detection winding 27 Output voltage detection circuit

Claims (1)

Claims: 1. When a switching element is turned on, a rectifying element conducts to supply energy from a secondary winding of a transformer to a smoothing choke coil. When the switching element is turned off, commutation occurs. A feedback control circuit that controls a pulse drive signal supplied to the switching element so that the element conducts and sends out the energy of the choke coil to the load side and stabilizes the output voltage supplied across the load. In the switching power supply device, a voltage detection winding is wound around the choke coil while being insulated from a main winding, and the feedback control is performed using a voltage generated in the voltage detection winding when the commutation element is turned on. A switching power supply device comprising an output voltage detection circuit for supplying a detection voltage to a circuit.